Drop Sync/DDOS Attack

1. Find.. to which IP address in the server is targeted by the ddos attack

netstat -plan  | grep  :80 | awk ‘{print $4}’ | cut -d: -f1 |sort |uniq -c

2. Find… from which IPs, the attack is coming

netstat -plan  | grep  :80 | awk ‘{print $5}’ | cut -d: -f1 |sort |uniq -c

In csf:

vi /etc/csf/csf.conf

SYNFLOOD is disabled by default. If you are not receiving any sort of attack, there is no need to enable it. If you are expecting an attack, enable it and set the rules a bit strict, like
SYNFLOOD_RATE = “5/s”
SYNFLOOD_BURST = “3”
my eg:
SYNFLOOD = “1”
SYNFLOOD_RATE = “30/s”
SYNFLOOD_BURST = “10”

SYNFLOOD

SYNFLOOD is disabled by default. If you are not receiving any sort of attack, there is no need to enable it. If you are expecting an attack, enable it and set the rules a bit strict, like

SYNFLOOD = “1”

SYNFLOOD_RATE = “30/s”

SYNFLOOD_BURST = “10”

i.e. if 30 connections are received from an IP/sec for 10 times, block it. Make sure don’t keep it too strict if you are not receiving an attack else it will generate false positives and will block legit connections.

PORTFLOOD

PORTFLOOD = 80;tcp;100;5,22;tcp;5;300

ie, If an IP makes 100 connections in 5 sec to port 80 (tcp), then it will be blocked from the server and if 5 connections in 300 sec to 22 port.


In /etc/sysctl.conf

Paste the following into the file, you can overwrite the current information.

#Kernel sysctl configuration file for Red Hat Linux

#

# For binary values, 0 is disabled, 1 is enabled. See sysctl(8) and

# sysctl.conf(5) for more details.


# Disables packet forwarding

net.ipv4.ip_forward=0


# Disables IP source routing

net.ipv4.conf.all.accept_source_route = 0

net.ipv4.conf.lo.accept_source_route = 0

net.ipv4.conf.eth0.accept_source_route = 0

net.ipv4.conf.default.accept_source_route = 0


# Enable IP spoofing protection, turn on source route verification

net.ipv4.conf.all.rp_filter = 1

net.ipv4.conf.lo.rp_filter = 1

net.ipv4.conf.eth0.rp_filter = 1

net.ipv4.conf.default.rp_filter = 1


# Disable ICMP Redirect Acceptance

net.ipv4.conf.all.accept_redirects = 0

net.ipv4.conf.lo.accept_redirects = 0

net.ipv4.conf.eth0.accept_redirects = 0

net.ipv4.conf.default.accept_redirects = 0


# Enable Log Spoofed Packets, Source Routed Packets, Redirect Packets

net.ipv4.conf.all.log_martians = 0

net.ipv4.conf.lo.log_martians = 0

net.ipv4.conf.eth0.log_martians = 0


# Disables IP source routing

net.ipv4.conf.all.accept_source_route = 0

net.ipv4.conf.lo.accept_source_route = 0

net.ipv4.conf.eth0.accept_source_route = 0

net.ipv4.conf.default.accept_source_route = 0


# Enable IP spoofing protection, turn on source route verification

net.ipv4.conf.all.rp_filter = 1

net.ipv4.conf.lo.rp_filter = 1

net.ipv4.conf.eth0.rp_filter = 1

net.ipv4.conf.default.rp_filter = 1


# Disable ICMP Redirect Acceptance

net.ipv4.conf.all.accept_redirects = 0

net.ipv4.conf.lo.accept_redirects = 0

net.ipv4.conf.eth0.accept_redirects = 0

net.ipv4.conf.default.accept_redirects = 0


# Disables the magic-sysrq key

kernel.sysrq = 0


# Decrease the time default value for tcp_fin_timeout connection

net.ipv4.tcp_fin_timeout = 15


# Decrease the time default value for tcp_keepalive_time connection

net.ipv4.tcp_keepalive_time = 1800


# Turn off the tcp_window_scaling

net.ipv4.tcp_window_scaling = 0


# Turn off the tcp_sack

net.ipv4.tcp_sack = 0


# Turn off the tcp_timestamps

net.ipv4.tcp_timestamps = 0


# Enable TCP SYN Cookie Protection

net.ipv4.tcp_syncookies = 1


# Enable ignoring broadcasts request

net.ipv4.icmp_echo_ignore_broadcasts = 1


# Enable bad error message Protection

net.ipv4.icmp_ignore_bogus_error_responses = 1


# Log Spoofed Packets, Source Routed Packets, Redirect Packets

net.ipv4.conf.all.log_martians = 1


# Increases the size of the socket queue (effectively, q0).

net.ipv4.tcp_max_syn_backlog = 1024


# Increase the tcp-time-wait buckets pool size

net.ipv4.tcp_max_tw_buckets = 1440000


# Allowed local port range

net.ipv4.ip_local_port_range = 16384 65536


Run /sbin/sysctl -p and sysctl -w net.ipv4.route.flush=1 to enable the changes without a reboot.


TCP Syncookies

echo 1 > /proc/sys/net/ipv4/tcp_syncookies


Some IPTABLES Rules:

iptables -A INPUT -p tcp –syn -m limit –limit 1/s –limit-burst 3 -j RETURN


iptables -A INPUT -p tcp –syn -m state –state ESTABLISHED,RELATED –dport 80 -m limit –limit 1/s –limit-burst 2 -j ACCEPT

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How to Boot second kernel if first fails

Reference :  http://www.linux-noob.com/forums/index.php?/topic/2928-grub-single-boot-and-kernel-panic-reboot/

If for example your grub.conf looks like the one i have below. This boots into the 2.6.20-1.2944 by default.

default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title Fedora Core (2.6.20-1.2944.fc6)
        root (hd0,0)
        kernel /vmlinuz-2.6.20-1.2944.fc6 ro root=LABEL=/ acpi=off
        initrd /initrd-2.6.20-1.2944.fc6.img
title Fedora Core
        root (hd0,0)
        kernel /vmlinuz-2.6.20 ro root=LABEL=/ acpi=off
        initrd /initrd-2.6.20.img

If you want to have it so that 2.6.20 boots on next boot, run the following command from the console before rebooting.
echo “savedefault –default=1 –once” | grub –batch
then
reboot

Now, if you have a kernel that panics and need it to reboot if it panics. change your config and add the panic=# (# being seconds before auto reboot) to the kernel line. Example file below

default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title Fedora Core (2.6.20-1.2944.fc6)
        root (hd0,0)
        kernel /vmlinuz-2.6.20-1.2944.fc6 ro root=LABEL=/ acpi=off panic=5
        initrd /initrd-2.6.20-1.2944.fc6.img
title Fedora Core (2.6.20)
        root (hd0,0)
        kernel /vmlinuz-2.6.20 ro root=LABEL=/ acpi=off panic=5
        initrd /initrd-2.6.20.img

so the order of operations on this are as follows.

1.) Add new kernel to grub.conf
2.) set default=# in grub.conf to the failsafe kernel (the one you want it to try if the other fails)
3.) run the following:
echo “savedefault –default=1 –once” | grub –batch
4.) reboot

Copied from :  http://www.linux-noob.com/forums/index.php?/topic/2928-grub-single-boot-and-kernel-panic-reboot/